Belgium Pat. No. 899,399 describes certain 7-piperazine-1-cyclopropyl-6,8-difluoro-1,4-dihydro-4-oxo-3-quinolinecarbo xylic acids. German Offenlegungschrift No. 3318145 describes various 7-amino-1-cyclopropyl-6,8-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acids. European Patent Publication No. 106489 describes 7-cyclic amine-1-cyclopropyl-6,8-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acids.
All of the above compounds are useful as antibacterial agents and have been described as being prepared by displacement of a 7-fluoro atom from a compound of the formula ##STR1## wherein X is hydrogen or fluorine; R.sub.1 is hydrogen or lower alkyl, and R.sub.2 is alkyl of one to three carbon atoms or cycloalkyl of three to six carbon atoms, with the appropriate amine.
The object of the present invention is an improved process for preparing the compounds described above by converting a lower alkyl ester of 1-alkyl or cycloalkyl-6,7,8-trifluoro or 6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid to a silyl ester thereof which can be directly used in the displacement reaction with the amine displacing the 7-fluoro to give the final product. The present method provides better quality material with fewer purification procedures, step-saving by being able to carry out the reaction in one-pot, and practically quantitative yields from the 1-alkyl or 1-cycloalkyl-6,7,8-trifluoro or 6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid alkyl esters.